He could go small and low but the number of drivers needed to get the efficiency required exceeds the budget. Low and loud but then he also goes above box size limits. loud and small but won't go flat to 30Hz.

It could be done with passive radiators but that's out...
The port needed would fill the box...
No horn could be that size and tuning for the required SPL...

If used in a room, you could go for two high-excursion drivers like the sonic-10 from Carpower (+-25mm xmax linear) und put them in a 40l sealed Box, which gives you this, standing on the floor:
Of course, three will do better... Some EQ, and you are there.